1. Field of the Invention
The present invention relates to throttle bodies that have throttle valves actuated by motors for controlling the rotational speed of internal combustion engines.
2. Description of the Related Art
There are known throttle bodies that have motor actuated throttle valves. For example, Japanese Laid-Open Patent Publication No. 2001-132495 teaches a throttle body in which a throttle valve, a throttle shaft, a motor, a gear mechanism, and a throttle sensor, are disposed. The throttle valve is secured to the throttle shaft and the motor rotatably drives the throttle shaft. The gear mechanism serves to transmit the driving force of the motor to the throttle shaft. The throttle sensor serves to detect a degree of opening of the throttle valve.
Additionally, in recent years in order to reduce the weight and the manufacturing costs of an automobile, there has been a tendency to use materials such as resin in the fabrication of automobiles parts, possibly including such parts as throttle bodies.
However, in the case of throttle bodies made of resin, there is a possibility that some problems may be caused by this material selection. These problems will be explained with reference to FIG. 4.
A conventional throttle body 61 is shown in FIG. 4 and includes a main body 62. The main body 62 has a bore wall portion 62a that defines an intake air channel, in which a throttle valve 5 is disposed. The main body 62 also defines a space for receiving a motor 11. In the case where the main body 62 is made of resin, the heat produced by the motor 11 may not be effectively dissipated to the outside of the main body 62, due to the low heat conduction efficiency of the material. Therefore, there is a possibility of overheating and damaging the motor 11. In addition, because the heat may not be effectively dissipated, the bore wall portion 62a may be thermally deformed and cause unwanted interference with the throttle valve 5. In such a situation, the controllability of the throttle valve 5 may be lessened. Further, if a molding process, such as an injection molding process, forms the main body 62, there is a possibility that attaining the substantial circularity of the intake air channel will be inhibited due to the variations in the thickness of the bore wall portion 62a along the circumferential length of the intake air channel. The resulting bore wall portion 62a due to molding conditions may also cause unwanted interference with the throttle valve 5.